Fluid flow in shear zones: insights from the geometry and evolution of ore bodies at Renco gold mine,Zimbabwe

The geometry of mineral deposits can give insights into fluid flow in shear zones. Lode gold ore bodies at Renco Mine, in the Limpopo Belt, Zimbabwe, occur as siliceous breccias and mylonites within amphibolite facies shear zones that dip either gently or steeply. The two sets of ore bodies formed synchronously from hydrothermal fluids. The ore bodies are oblate, but have well‐defined long axes. Larger ore bodies are more oblate. High‐grade gold ore shoots have long axes that plunge down dip; this direction is perpendicular to the long axes of the low‐grade ore bodies. The centres of the high‐grade ore bodies align within the low‐grade ore bodies along strike in both gently and steeply dipping groups. The range of sizes and shapes of the ore bodies are interpreted as a growth sequence. Geometrical models are proposed for the gently and steeply dipping ore bodies, in which individual ore bodies grow with long axes plunging down dip, and merge to form larger, more oblate ore bodies. The models show that when three or more ore bodies coalesce, the long axis of the merged ore body is perpendicular to the component ore bodies, and that ore bodies in the deposit may have a range of shapes due to both growth of individual ore bodies, and their coalescence. The long axes of the high‐grade ore bodies are parallel to the shear directions of both the gently and steeply dipping dip slip shear zones, which were the directions of greatest permeability and fluid flow. The larger, lower grade bodies, which may have formed by coalescence, are elongate perpendicular to these directions.     

Simple Seismic Assessment of Traditional Unreinforced Masonry Buildings

A framework for quick seismic assessment and retrofit of traditional unreinforced masonry (URM) structures is presented. The proposed methods build on simple principles of structural dynamics and are used as an alternative to detailed time-history analysis, in recognition of the prevailing need for simple and practical methods, compatible with the low-budget and the limited level of knowledge regarding materials, internal force paths, connectivity and condition of older URM structures. An objective is to identify areas in the building that are particularly susceptible to damage and for guiding the types of the required global interventions to improve seismic response. Demand and supply are expressed in terms of relative drift ratios that quantify the intensity of out-of-plane differential translation and in-plane shear distortion of masonry walls. A characteristic traditional building type of timber-laced masonry is used as a model structure for illustration of concepts. The morphology and geometry of the building correspond to a statistical sample of the actual traditional unreinforced masonry buildings (TURM) found in historical centers of many towns in the greater region of Northern Greece. The methodology is particularly useful for setting retrofit priorities and management of the collective seismic risk of historical entities.     

The role of the stress regime on microseismicity induced by overpressure and cooling in geologic carbon storage

Fluid injection in deep geological formations usually induces microseismicity. In particular, industrial‐scale injection of CO₂ may induce a large number of microseismic events. Since CO₂ is likely to reach the storage formation at a lower temperature than that corresponding to the geothermal gradient, both overpressure and cooling decrease the effective stresses and may induce microseismicity. Here, we investigate the effect of the stress regime on the effective stress evolution and fracture stability when injecting cold CO₂ through a horizontal well in a deep saline formation. Simulation results show that when only overpressure occurs, the vertical total stress remains practically constant, but the horizontal total stresses increase proportionally to overpressure. These hydro‐mechanical stress changes result in a slight improvement in fracture stability in normal faulting stress regimes because the decrease in deviatoric stress offsets the decrease in effective stresses produced by overpressure. However, fracture stability significantly decreases in reverse faulting stress regimes because the size of the Mohr circle increases in addition to being displaced towards failure conditions. Fracture stability also decreases in strike slip stress regimes because the Mohr circle maintains its size and is shifted towards the yield surface a magnitude equal to overpressure minus the increase in the horizontal total stresses. Additionally, cooling induces a thermal stress reduction in all directions, but larger in the out‐of‐plane direction. This stress anisotropy causes, apart from a displacement of the Mohr circle towards the yield surface, an increase in the size of the Mohr circle. These two effects decrease fracture stability, resulting in the strike slip being the least stable stress regime when cooling occurs, followed by the reverse faulting and the normal faulting stress regimes. Thus, characterizing the stress state is crucial to determine the maximum sustainable injection pressure and maximum temperature drop to safely inject CO₂.     

ENERGY-BASED TECHNIQUE FOR SEISMIC PERFORMANCE ASSESSMENT OF INTERIOR BEAM-COLUMN JOINTS

For investigating the seismic behaviour of monolithic beam-column joints, a new technique of assessment of joint performance, termed the Shear Deformation Energy Index, is presented in this paper. Previous research efforts in this field were evaluated and the relative merits and drawbacks of each approach were highlighted. Primary variables influencing the seismic behaviour of joints were studied in the experimental phase of the current study. This included pseudo-static testing of nine interior beam-column sub-assemblages with different joint configurations. The parametric investigation of primary variables was complemented by studying the behaviour of specimens of eight different experimental programmes. For easy application of the technique in design practice, the Index is graphically represented versus its main variables in a design chart, referred to as the Performance Chart.     

NONLINEAR ANALYSIS OF REINFORCED CONCRETE SHEAR WALL UNDER EARTHQUAKE LOADING

A constitutive model for predicting the cyclic response of reinforced concrete structures is proposed. The model adopts the concept of a smeared crack approach with orthogonal fixed cracks and assumes a plane stress condition. Predictions of the model are compared firstly with existing experimental data on shear walls which were tested under monotonic and cyclic loading. The same model is then used in the finite element analysis of a complete shear wall structure which was tested under a large number of cyclic load reversals due to earthquake loading at NUPEC's Tadotsu Engineering Laboratory. Two different finite element approaches were used, namely a two-dimensional and a three-dimensional representation of the test specimen. The ability of the concrete model to -reproduce the most important characteristics of the dynamic behaviour of this type of structural element was evaluated by comparison with available experimental data. The numerical results showed good correlation between the predicted and the actual response, global as well as local response being reasonable close to the experimental one.     

PREDICTION OF DAMAGE IN R/C SHEAR PANELS SUBJECTED TO REVERSED CYCLIC LOADING

In this paper, the damage prediction of shear-dominated reinforced concrete (RC) elements subjected to reversed cyclic shear is presented using an existing damage model. The damage model is primarily based on the monotonic energy dissipating capacity of structural elements before and after the application of reversed cyclic loading. Therefore, it could be universally applicable to different types of structural members, includeing shear-dominated RC members. The applicability of the damage model to shear-dominated RC members is assessed using the results from reversed cyclic shear load tests conducted earlier on eleven RC panels. First, the monotonic energy dissipating capacities of the panels before and after the application of reversed cyclic loading are estimated and employed in the damage model. Next, a detailed comparison between the analytically predicted damage and the observed damage from the experimental tests of the panels is performed throughout the loading history. Subsequently, the effects of two important parameters, the orientation and the percentage of reinforcement, on the damage of such shear-dominated panels are studied. The research results demonstrated that the analytically predicted damage is in reasonably good agreement with the observed damage throughout the entire loading history. Furthermore, the orientation and percentage of reinforcement is found to have considerable effect on the extent of damage.     

An Enhanced Finite Element Macro-Model for the Realistic Simulation of Localized Cracks in Masonry Structures: A Large-Scale Application

ABSTRACT

Finite element macro-modeling approaches are widely used for the analysis of large-scale masonry structures. Despite their efficiency, they still face two important challenges: the realistic representation of damage and a reasonable independency of the numerical results to the used discretization. In this work, the classical smeared crack approach is enhanced with a crack-tracking algorithm, originating from the analysis of localized cracking in quasi-brittle materials. The proposed algorithm is for the first time applied to a large-scale wall exhibiting multiple shear and flexural cracking. Discussion covers structural aspects, as the response of the structure under different assumptions regarding the floor rigidity, but also numerical issues, commonly overlooked in the simulation of large structures, such the mesh-dependency of the numerical results.     

Shear Vulnerability of Historical Reinforced-Concrete Structures

Modern buildings have been characterized by the rapid spread of reinforced concrete as a novel and versatile building material. Within the assessment of existing buildings, the most sensitive structures are those designed in the first half of the 20th century, when the theory of reinforced concrete and the detailing rules were not yet well established. This study considers the issues related to the gradual understanding of the vulnerability at a territorial scale of structural typologies designed only for vertical loads. In particular, the shear behavior of the buildings in the period between 1920 and 1960 is investigated. The shear strength control of the structural elements of an open-air stadium in Southern Italy is carried on according to Eurocode 2, the American Concrete Institute Code, ACI 318R–2008, and a method based on Arslan’s equation. The results are compared with those included in the original calculus report according to the 1930 Royal Decree-Law (RDL 18/7/1930).     

Rates of retrograde metamorphism and their implications for crustal rheology

The rate of reaction of a natural hornblende garnet granulite with water under a range of mid- to lower crustal conditions has been investigated experimentally. In runs of between 7 and 84 days small but measurable amounts of water were consumed, and sheet silicates (300°C, 300 MPa and 400°C, 400 MPa) and/or secondary actinolite (400°C, 400 MPa and 500°C, 500 MPa) were observed to have grown. When normalized to the surface area of the starting materials, hydration rates were in the range of 2–5 × 10⁻⁸ g m⁻² sec⁻¹. These reaction rates imply that a film of water that infiltrated a planar crack with a half width of 100 μm would be completely consumed within c. 100 years. These results imply that where water infiltrates the crust along faults or underlying shear zones in response to a deformation, it will remain as a free phase for only a finite period of time, which in some cases will be less than the repeat time for major earthquakes in the fault system. Thus, the rheology of fault zones and shear zones is likely to be cyclical, with the zone becoming stronger with time as water is consumed, and then weakened by infiltration of water after each rupture.     

The evolution of strength,elasticity and rupture behaviour of birch tar made with ‘double-pot’ techniques during tar cooking

Birch tar was the first adhesive produced by humans. Its study has consequences for our understanding of human evolution and the development of specialised craftsmanship. One of the better-documented birch tar making methods is the ‘double-pot’ technique, where two containers are used, one containing bark, the other collecting the tar. Birch tar made with double-pots has low viscosity and bad adhesive properties. To obtain a usable adhesive, it must be reduced in volume by cooking. We investigate the evolution of tar’s mechanical properties during cooking. We use lap-shear tests to investigate strength, stiffness and failure behaviour. We found that tar must be cooked for 90 min to acquire adhesive properties. When cooked for longer, strength and stiffness increase over a 30 min lasting time span. Cooking for even longer, beyond this 30 min window, produced a substance that could no longer be used as an adhesive. The implications are that tar cooking requires a high level of skill because specific signs indicating the desired properties must be recognised during the process. Tar cooking constitutes a supplementary investment in time and resources and appears to be associated with a certain degree of risk.